ABSTRACT: The effects of solar UVA radiation on total bacterial production (3H-thymidine [TTI] and 14C-leucine [TLI] incorporation), bacterial numbers, chlorophyll a, algal biomass and photosynthesis–irradiance (P–E) response curves in Baltic Sea land-fast ice were studied in situ in the Gulf of Finland in March 2005. Two tent-shaped frames, one covered with UVR opaque foil (exposure to photosynthetically active radiation [PAR]) and the other with UVA transparent foil (exposure to PAR+UVA), were set up on snow-free sea ice. The ice was incubated for 21 d, and weekly samples were taken from both treated and snow-covered untreated (UNT) ice. The exposure to incident PAR controlled the algal biomass but exposure to UVA caused an additional decrease in biomass (mean UNT, 47 µg C l–1; PAR, 38 µg C l–1; PAR+UVA, 18 µg C l–1). Unidentified flagellates <20 µm were more sensitive to PAR than to UVA, whereas the other algal groups were more sensitive to UVA (chlorophytes, pennate diatoms) or mutually sensitive to PAR and UVA (centric diatoms). In the top 10 cm, the higher values of photosynthetic parameters (αb and Pbm) in the PAR+UVA compared with the PAR treatment reflected the differences in the algal community composition. The effects of UVA on bacterial production were mainly limited to the top 5 cm and were linked to changes not only in algal biomass but also in algal species composition. After the first incubation week, the TLI:TTI ratios in the clearly flagellate-dominated surface layers (UNT, PAR+UVA) were higher than in the PAR treatment, where the increase in TTI was attributed to the marked increase in chlorophyte and pennate diatom biomass. These results show that the responses of Baltic sea-ice organisms must be taken into account when the effects of climate change are assessed.